Methods and devices for reading data from non-volatile memory cells

ABSTRACT

Disclosed is a method for responding to a single user read command of a complementary cell array including one or more complementary cell pairs, the method including: determining if a first group of cells out of a data word is in an erased state or in a programmed state, and outputting a data word so that (a) if the first group of cells is determined to be erased a logical “one” is output for each bit of the data word and (b) if the first group of cells is determined to be programmed the result of a complementary read is output for each bit of the data word.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a continuation of U.S. patent application Ser. No.15/279,974, filed Sep. 29, 2016, which claims the priority and benefitof U.S. Provisional Application No. 62/351,183, filed on Jun. 16, 2016,all of which are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to the field of Non-VolatileMemory (NVM) devices. More specifically, the present invention relatesto methods, circuits, devices, and systems for reading data from NVMcells.

BACKGROUND

Integrated circuits (IC) may be used in a wide range of designs andproducts, some integrated circuits may include Non-Volatile Memory (NVM)arrays. An NVM array may be composed of NVM cells, ancillary circuitry,a controller and additional circuits. The ancillary circuitry mayinclude for example: array controls, address decoding circuits and senseamplifiers (SA) and/or comparators. SAs may be configured to determine avalue/level of one or more targeted NVM cell.

Some memory array types may include NVM arrays, floating gate arrays,array of eCT cells, array of MirrorBit cells, charge trapping cells andmore.

The NVM cells may be single bit or multi-level storage cells, and thecells may be programmable to different states. For example, in a singlebit configuration the cell may be programmable to either an erased (ERS)or programmed (PRG) state.

According to some embodiments, the NVM cells may be accessed internallyin the IC through wordlines (WLs), bitlines (BLs), select lines (SLs),memory gates (MGs) or otherwise. For each sequence (Programming ofcells, Erasing of cells, Reading of cells etc.), the WLs, BLs, SLs,and/or MGs may be activated accordingly. Operating mode, such as readalgorithm, program algorithm, erase algorithm, may determine voltage orcurrent signals applied to WLs, SLs, SGs, MGs of NVM cells. Otherfactors include the selected addresses, the specific technology beingused. In some embodiments, the NVM arrays may include differentstructural features and may not include select gates (SGs), BLs, SLs,MGs and/or WLs, or otherwise.

Some transistor types which may be used in associated circuitry arePmos, Nmos, low voltage (LV) Nmos, LV Pmos, high voltage (HV) Nmos andHV Pmos, Zmos which is a low resistance Nmos or Pmos transistor, bipolarjunction transistor (BJT), and more. HV transistors/cells may bedifferentiated from LV transistors/cells by being designed/configured toenable operation in a higher range of voltages across their channelcompared to LV cells (for example, between a drain node and a sourcenode of the transistor) and/or across the gate (for example: betweentheir gate and bulk or ground node), and may include a thick oxideregion compared to LV devices.

Externally, a host device may including an IC and/or a memory arraywhich may be accessed by a user/host device by sending a user/hostdevice command to carry out a User Read operation, User Programoperation, User Erase Operation or otherwise and may include an addressthat is requested to have the operation applied to. In response to theuser Read command, information from the memory device may be output tothe user/host device. In response to a User Program command, informationmay be stored in the memory device. In response to a User Erase command,a segment of the memory may be erased.

SUMMARY OF THE INVENTION

The present invention may include methods, circuits, devices and systemsfor storing and reading data from non-volatile memory (NVM) cells.

According to some embodiments, a method for responding to a single userread command of a complementary cell array including one or morecomplementary cell pairs may include: determining if a first group ofcells out of a data word is in an erased state or in a programmed state,and outputting a data word so that (a) if the first group of cells isdetermined to be erased a predetermined binary state (such as “one” or“zero”) is output for each bit of the data word and (b) if the firstgroup of cells is determined to be programmed the result of acomplementary read is output for each bit of the data word.

According to some embodiments, determining if a first group of cells isin an erased state or in a programmed state may further include countinghow many of the first group of cells are in a first binary state toproduce a count value and comparing the count value to a predeterminedthreshold. The predetermined threshold may be programmable and may beupdated based on at least one of the following parameters: charge loss,length of life, array cycle count, beginning of life and end of life andmore.

According to some embodiments the method comparing a count value to thepredetermined threshold may result in/produce a state result. If a firstbinary state is ‘0’ then the state result may be (a) an erased state ifthe count value is below the predetermined threshold and (b) the stateresult may be a programmed state if the count value is above or equal tothe predetermined threshold. Or, if the first binary state is ‘1’ thenthe state result may be (a) an erased state if the count value is abovethe predetermined threshold and (b) the state result may be a programmedstate if the count value is below or equal to the predeterminedthreshold.

According to some embodiments, counting may be stopped if the countvalue exceeds the predetermined threshold. Furthermore, determining if afirst group of cells is in an erased state or in a programmed state mayinclude a first read event to determine a value of each of a sub-groupof target cells out of the first group of cells. Optionally, determiningif a first group of cells is in an erased state or in a programmed statefurther includes a second read event to determine a value of each of asub-group of twin cells out of the first group of cells. The first andsecond read events may be carried out simultaneously or serially.

According to some embodiments, an NVM device may include: (a) an NVMarray including one or more complementary cell groups each groupincluding at least one pair of cells including a target and twin cellthat when in a programmed state the target and twin cells are inopposing binary states, (b) a first plurality of sense amplifiers todetect a state of a first plurality of cells and to output anappropriate binary value, (c) a counter to produce a sum of the binaryvalues received from the sense amplifiers, (d) a controller to determineif a data word is in an erased or programmed state, and (e) astate-dependent switch to output a data word so that if the word isdetermined to be erased a predetermined binary value (“one” or “zero”)is output for each bit of the data word (the same binary value is outputfor all of the bits) and if the word is determined to be programmed theresult of a complementary read is output for each bit of the data word.

According to some embodiments, an NVM device may include logic circuitryto compare a count value to a predetermined threshold and produce astate result.

According to some embodiments, the controller may be configured todetermine if the data word is in an erased or programmed state based onthe state result. Accordingly, if a first binary state is ‘0’, and ifthe count value is below the predetermined threshold an erased state isdetermined. The word may be determined to be in a programmed state ifthe count value is above or equal to the predetermined threshold.

According to some embodiments, a first plurality of cells may bereceived from a plurality of target cells and each of a first pluralityof sense amplifiers may be configured to compare a target cell and areference signal and to relay to the counter the binary state of thetarget cell.

According to some embodiments, a first plurality of cells may bereceived from a plurality of twin cells and each of the first pluralityof sense amplifiers may compare a twin cell and a reference signal andaccordingly relay to the counter the binary state of the twin cell.Optionally, the first plurality of sense amplifiers may each receive atwin cell and a target cell and determine the binary value of the targetcell.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIG. 1, shown is a flow chart for a method for implementing a singleread access according to one embodiment of the subject matter;

FIG. 2, shown is a flow chart for a method for determining if a word isprogrammed or erased;

FIG. 3A depicts an example block diagram of an NVM device including anNVM array;

FIG. 3B depicts an example NVM array and associated sense amplifiers(SAs); and

FIGS. 4A-4C depict different SA connectivity for different processes ofthe same user read command.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity. Further, where consideredappropriate, reference numerals may be repeated among the figures toindicate corresponding or analogous elements.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components and circuitshave not been described in detail so as not to obscure the presentinvention.

Unless specifically stated otherwise, as apparent from the followingdiscussions, it is appreciated that throughout the specificationdiscussions utilizing terms such as “processing”, “computing”,“calculating”, “determining”, or the like, refer to the action and/orprocesses of circuits, storage, transmission or display devices thatmanipulate and/or transform data represented as physical, such aselectronic, quantities within the circuits into other data or other suchinformation.

Embodiments of the present invention may include apparatuses forperforming the operations herein. This apparatus may be speciallyconstructed for the desired purposes. The processes and displayspresented herein are not inherently related to any particular computeror other apparatus. Various general purpose systems may be used withprograms in accordance with the teachings herein, or it may proveconvenient to construct a more specialized apparatus to perform thedesired method. The desired structure for a variety of these systemswill appear from the description below.

NVM array programming circuits, integral or otherwise functionallyassociated with an NVM array may include a complementary pair includinga target and twin cell which may be marked as bit or true bit andcomplementary bit or cbit, respectively. A complementary pair of NVMcells may store one bit of data on two separate but complementary NVMcells, as inverse or opposite NVM cell charge states on each of the twocomplementary NVM cells. Similarly, multiple complementary cells arealso understood, so that one bit of data may be stored on 4, 6, 8 ormore cells. Multiple complementary cells may be implemented by severalmethods, including but not limited to, averaging the cells' signals pereach complementary value by using a majority decision or otherwise.Increasing the number of cells per bit may improve reliability whileincreasing die size or decreasing the total amount of data that may bestored in the memory. For clarity, in this application, a pair of cellsper one data bit will be discussed. However, changing specificembodiments to accommodate multiple pairs of complementary memory cellsper bit would be understood by those skilled in the art.

According to some embodiments, a data bit from a pair of complementaryNVM cells may be detected using a complementary read sequence usingdifferential sensing. According to the complementary read sequence,current from one NVM cell (such as the twin cell) of the NVM cell pairmay be used as a reference current when reading a current level from theother NVM cell (such as the target cell) of the NVM cell pair.Additional complementary sensing methods are understood, such ascomparing voltages of a target cell to a twin cell or otherwise. Duringan Erase sequence, the NVM array cell formatting circuits, integral orotherwise functionally associated with the NVM array, may erase ordischarge each of the NVM cells of a complementary NVM cell pair.Optionally, it may perform erase or discharge operations on entiresectors or blocks of NVM cells at the same time. Accordingly, an erasesequence may cause all of the memory cells (targets and twins) to beerased, accordingly for a cell pair both target and twin cell will be inthe same (erased) state.

Accordingly, if a user read command is received on an erased sector, inprior art solutions, this may result in corrupt information. If acomplementary read is carried out, two erased cells (target and twin)may be compared to each other. Since there may be some variation involtage or current between them, the bit may be detected as having aprogrammed state despite the whole sector being erased. In someembodiments, it may involve receiving a user read access and then theuser is required to check if the information is corrupt, or not. It mayalso require the user to monitor or know if a requested sector iserased, and if a read command is received for an erased sector corruptinformation will be provided. Accordingly, there is a need for a methodfor implementing a simple single-read access that outputs correct(non-corrupted) data regardless if the word or NVM array is erased orprogrammed.

Turning to FIG. 1, shown is a flow chart of a method for implementing asingle read access 100. A memory device may receive a user read command(102) from a user (or user device) and may then determine if therequested word is erased or programmed (step 104). If the word isdetermined to be erased, a word consisting of bit(s) of a single binaryvalue (all ‘1’s or all “0”s) may be output to a user (step 106). If aword is determined to be programmed, the result of a complementary reador differential sensing may be output to the user (step 108).Accordingly, it is ensured that non-corrupt information is supplied to auser in response to a single user-read access/request. Furthermore,typically a “1” is utilized to indicate an erased state. However this isarbitrary, and if selected during design, a “0” may be utilized toindicate an erased bit.

While method 100 has been described by way of example for a word, it isunderstood that each of the steps 104-108 may be carried out on a wordor sector which may include a group of words or otherwise, depending onthe array architecture and other considerations.

Turning to FIG. 2, a flow chart for a method 200 for determining if aword is programmed or erased is shown. It is understood that method 200is substantially similar to step 104 of FIG. 1. According to someembodiments, a first group of target cells that store bit may bedetected (step 202) as well as a second group of twin cells that storecbit (step 204). A reference read sequence may be utilized for detectingthe first and second groups of memory cells. The reference read sequencemay include comparing the cell value to a reference cell value, a staticreference cell, dynamic reference cell, current or voltage reference, acombination of these and more, and providing a binary value for thecharge stored on the cell (typically ‘1’ for an erased cell and ‘0’ fora programmed cell).

According to some embodiments, an N bit data word (referred to herein asa word) may consist of 2N data cells (N cell and N complementary cells).The first group of target cells and second group of twin cells may eachbe N or less and may be equal or different in size. Furthermore,detection of the target and twin cells may be carried out in parallel,serially, or interleaved depending on the amount of senseamplifier/comparator, power considerations and more.

According to some embodiments, detecting a cell may include attributinga binary state to the cell based on a detected signal from the cell.Typically, as is customary in the art, an erased cell may be consideredas having a binary value of ‘1’, and a programmed cell may be consideredas have a binary value of ‘0’. These are only conventions, and of courseit is possible to attribute oppositely.

According to some embodiments, the binary values of the first and secondgroups of cells may be counted, this may also be termed “zero counting”.In one embodiment, the number of programmed bits may bedetected/counted/summed resulting in a count value (step 206). It isunderstood that essentially what is being counted is the amount of bitshaving a first predetermined binary state, and which state that is(erased or programmed) is dependent on the convention selected (erase is“1” or “0” etc.).

According to some embodiments, a count value may be compared to apredetermined state-parameter threshold (step 208), resulting in a stateresult. According to some embodiments, if the total number of programmedcells (“0”) is equal to or exceeds the state-parameter threshold, theword may be determined to be in a “programmed” state (step 210).Otherwise, the state of the word may be determined to be in an “erased”state (step 212). However, since the determination is arbitrary (that aprogrammed cell is ‘0’ and an erased cell is ‘1’), it is clear to thoseskilled in the art that other configurations are applicable (so that ifthe number of programmed cells is below the state-parameter thresholdthe word may be determined to be programmed).

According to some embodiments, a word programmed state indicates thatthe word has been through a program sequence and accordingly, thecomplementary bits are in a complementing state, i.e. each pair of cellsare in opposite binary states (if a target cell is in a programmed statethe twin cell is in an erased state, and vice versa). A word erasedstate indicates that the word has gone through an erase sequence. In aword erased state, all of the cells are in an erased state so thatcomplementary bits are not in a complementing state. i.e. each pair ofcells is in the same binary state (typically ‘1’).

It is understood, that single or even multiple failures of detection ofthe target and/or twin cells compared to a reference would not impactthe method's validity or result because of the large difference betweenthe number of expected 0's for an erased state (none) compared to aprogrammed state (half of the total amount of cells, target and twin).This very large window allows a high reliability operation of the systemutilizing the method described herein.

According to some embodiments, the level of the programmable predefinedthreshold may be updated/optimized to accommodate charge loss or othernon-ideal effects, so that the method would remain operable for bothbeginning of life and end of life operation.

Accordingly, the predetermined threshold may be a function of the numberor a total count of expected binary value (for example 0's) for aprogrammed word. If all of the target and all of the twin cells arebeing detected, for a word of length N bits, there are 2N cells. In oneembodiment, N cells would be detected as having a binary value of ‘0’(in this example) for a programmed cell (since either the target or thetwin would be ‘0’ in a programmed complementary cell). Accordingly, athreshold of N/2 or N/3 or even N/4 or less may be operable.Furthermore, it is understood that the cells may be detected or readuntil the threshold is exceeded. Then, detection and/or analysis may bestopped (depending on array architecture and design).

According to some embodiments, the predetermined state parameterthreshold may be programmable. It may also be updated based on, forexample, at least one of the following parameters: charge loss, lengthof life, array cycle count, beginning of life and end of life. Chargeloss may include retention loss of charge over time or over a devicebeing cycled (i.e. other sections/words being erased and programmed).Length of life may also be a parameter relevant to physicalcharacteristics of the NVM cells and charge stored thereon. Array cyclecount is the amount of cycles, or expected cycles applied to the array.Beginning of life and end of life are well known terms in the artrelating to different typical behaviors of NVM cells at different stagesof a device cycle (first time a cell is being programmed vs. after agreat number of program and erase cycles).

Turning to FIG. 3A, depicted is an example block diagram of an NVMdevice 300. NVM device 300 includes a memory array such as array 302 forstoring information/data. Adjacent or included within array 302, andfunctionally associated with array 302, is a plurality of comparators orsense amplifiers such as SA₁-SA_(n) (304 ₁-304 _(n) appropriately).Array 302 may include a plurality of NVM cells and peripheral circuitryas described in this application and known in the art. Furthermore, theNVM cells of array 302 may be in a complementary configuration so thateach target cell has a twin cell and at least two cells are assigned perbit (true bit and complementary bit). SAs 304 ₁-304 _(n) may beconfigured to receive two signals, compare between them and output abinary value based on the comparison between the two signals. In oneembodiment, the two signals may be bit from target cell and a referencesignal, or cbit from twin cell and a reference signal. SAs 304 ₁-304_(n) may be functionally connected to binary state counter 306 which maycount how many of the SAs results are of a specific binary value (maycount ‘0’s or ‘1’s), and may output a state indicator indicating if thecounted binary values are above or below a predetermined threshold toindicate if the word is in an erased or programmed state. The countvalue may be utilized as a control signal of a state dependent switchsuch as mux₁-mux_(n) (308 ₁-308 _(n) appropriately) and cause either acomplementary read or differential sensing result to be output to a user(if a programmed state is indicated) or a logical ‘1’ be output to theuser (if an erased state is indicated).

It is understood that SAs 310 ₁-310 _(n) may be the same SAs as SAs 304₁-304 _(n), or not, depending on the array architecture and amount ofSAs available.

Turning to FIG. 3B, depicted is an example array 350 which may includearrays or sector of array 352 and a plurality of SA such as SA₁-SA_(s)(elements 354 ₁-354 _(s)) and that the amount of SAs may be equal ordifferent than the amount of bits ‘n’. It is understood that array 352and SA 354 ₁-354 _(n) are substantially similar to array 302 and SA 304₁-304 _(n) of FIG. 3A. Array 352 may include a plurality of words suchas words 356 ₁-356 _(m). Each word may include a plurality ofcomplementary cells, each pair including a target (B_(i)) and a twin(cB_(i)).

Turning to FIGS. 4A-4C depicted are different connectivity for differentprocesses of the same user read command. In FIGS. 4A and 4B connectivityappropriate for a reference read sequence is shown. Accordingly, thetarget and twin cells which are each being detected separately so thatthe total number of cells in a specific binary state (‘1’ or ‘0’) can becounted. Accordingly, in FIG. 4A, a bit cell is compared to a referenceand in FIG. 4B a twin cell is compared to a reference. In FIG. 4C theconnectivity is appropriate for a complementary read sequence, the SA isconnected so that the target cell is compared to its twin cell. In oneembodiment, a bit value is produced so that data can be output to a userif a programmed state is detected. It is understood that the SAs ofFIGS. 4A-4C may be the same SA (so that a single SA is used to for allthree comparisons sequentially) or the SA may be different SAs (so thatthe comparisons may be carried out at least partially in parallel), orany combination of these two options.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those skilled in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

What is claimed is:
 1. A method, comprising: obtaining a memory deviceincluding N complementary memory cells, each complementary memory cellincluding a first and a second memory cells, wherein N is a naturalnumber greater than 0, and wherein the memory device stores a thresholdnumber that is pre-defined initially and programmable subsequently;determining if each of the first memory cells of the N complementarymemory cells is in a first binary state or a second binary state;determining if each of the second memory cells of the N complementarymemory cells is in the first binary state or the second binary state;generating a count value by counting a total number of the first andsecond memory cells that are in the first binary state; and determiningif the N complementary memory cells are programmed based on a result ofcomparing the count value to the threshold number.
 2. The method ofclaim 1, wherein the first binary state represents data “0” and thesecond binary state represents data “1”.
 3. The method of claim 1,wherein the first binary state represents data “1” and the second binarystate represents data “0”.
 4. The method of claim 1, further comprising:updating the threshold number during a life of the memory device toaccommodate charge loss such that the N complementary memory cells areoperable from beginning to end of life of the memory device.
 5. Themethod of claim 1, wherein the first memory cells are bit cells and thesecond memory cells are complementary cells.
 6. The method of claim 5,wherein when the N complementary memory cells to be determinedprogrammed, further comprising: outputting data content of the bit cellsas a result of a read command.
 7. The method of claim 4, wherein thethreshold number is updated based on at least one of: charge loss,length of life, cycle count, and a state of beginning or end of life ofthe N complementary memory cells.
 8. The method of claim 1, whereindetermining if each of the first memory cells is in the first or secondbinary states includes: detecting output of each of the first memorycells; comparing, using at least one first sense amplifier, the outputof each of the first memory cells to a reference signal.
 9. The methodof claim 8, wherein determining if each of the second memory cells is inthe first or second binary states includes: detecting output of each ofthe second memory cells; comparing, using at least one second senseamplifier, the output of each the second memory cells to the referencesignal.
 10. The method of claim 6, wherein the output of the bit cellsof the N complementary memory cells include an N-bit word.
 11. Themethod of claim 9, wherein the reference signal is configured in a rangebetween a first binary state signal and a second binary state signal,and the threshold number is between 0 and N.
 12. A memory device,comprising: a plurality of complementary memory cells, each including abit cell and a complementary cell; a plurality of sense amplifiersconfigured to compare output of the bit and complementary cells to areference signal; a binary state counter coupled to the plurality ofsense amplifiers, configured to generate a count number corresponding toa cumulative number of the bit and complementary cells that includeoutput less than the reference signal, wherein the binary state counteris further configured to generate a state indicator based on acomparison between the count number and a state-parameter thresholdnumber that is pre-determined initially, wherein the state-parameterthreshold number is configured to be programmable and adjustablethroughout a life of the memory device; a plurality of multiplexors,wherein each multiplexor is configured to be coupled to a differentialsensing result of one of the plurality of complementary memory cells andan erased binary state signal, each multiplexor is further configured tobe coupled to the state indicator as a control signal.
 13. The memorydevice of claim 12, wherein the plurality of complementary memory cellsincludes N complementary memory cells, and the state-parameter thresholdnumber is pre-determined initially between 0 to N, wherein N is anatural number greater than
 0. 14. The memory device of claim 12,wherein the differential sensing result includes a comparison result ofthe output of the bit and complementary cells of a same complementarymemory cell.
 15. The memory device of claim 12, wherein thestate-parameter threshold number is updated based on at least one of thefollowing parameters: charge loss, length of life, array cycle count,and a state of beginning of life or end of life of the plurality ofcomplementary memory cells.
 16. A non-transitory computer readablestorage medium comprising stored instructions thereon, the instructionswhen executed cause a processing element to: execute a read command of Nmemory cells of a memory device, wherein N is a natural number greaterthan 0, and each memory cell includes a bit cell and a complementarycell, the read command further includes, a first read event whereinoutput of each of the bit cells is compared to a reference signal, and asecond read event wherein output of each of the complementary cells iscompared to the reference signal; generate a cumulative count valuecorresponding to results of the first and second read events; comparethe cumulative count value to a threshold value to determine if the Nmemory cells are in a programmed state, wherein the threshold value ispre-determined initially and programmable; and output results ofdifferential sensing of each memory cell when the N memory cells aredetermined in the programmed state.
 17. The non-transitory computerreadable storage medium of claim 16, wherein the first and second readevents are executed separately.
 18. The non-transitory computer readablestorage medium of claim 16, wherein the first and second read events areexecuted at least partially concurrently.
 19. The non-transitorycomputer readable storage medium of claim 16, wherein the results ofdifferential sensing of each memory cell of the N memory cells includean N-bit word.
 20. The non-transitory computer readable storage mediumof claim 16, wherein the threshold value is configured to be updatedduring a life of memory device such that the N memory cells are operablefrom beginning to end of life of the memory device.